package com.zx.sms.common;

/*
 * Copyright (C) 2012 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;

import java.util.BitSet;

/**
 * An immutable version of CharMatcher for smallish sets of characters that uses a hash table
 * with linear probing to check for matches.
 *
 * @author Christopher Swenson
 */
@GwtIncompatible("no precomputation is done in GWT")
final class SmallCharMatcher extends CharMatcher.FastMatcher {
    static final int MAX_SIZE = 1023;
    private final char[] table;
    private final boolean containsZero;
    private final long filter;
    
    private SmallCharMatcher(char[] table, long filter, boolean containsZero,
                             String description) {
        super(description);
        this.table = table;
        this.filter = filter;
        this.containsZero = containsZero;
    }
    
    private static final int C1 = 0xcc9e2d51;
    private static final int C2 = 0x1b873593;
    
    /*
     * This method was rewritten in Java from an intermediate step of the Murmur hash function in
     * http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp, which contained the
     * following header:
     *
     * MurmurHash3 was written by Austin Appleby, and is placed in the public domain. The author
     * hereby disclaims copyright to this source code.
     */
    static int smear(int hashCode) {
        return C2 * Integer.rotateLeft(hashCode * C1, 15);
    }
    
    private boolean checkFilter(int c) {
        return 1 == (1 & (filter >> c));
    }
    
    // This is all essentially copied from ImmutableSet, but we have to duplicate because
    // of dependencies.
    
    // Represents how tightly we can pack things, as a maximum.
    private static final double DESIRED_LOAD_FACTOR = 0.5;
    
    /**
     * Returns an array size suitable for the backing array of a hash table that
     * uses open addressing with linear probing in its implementation.  The
     * returned size is the smallest power of two that can hold setSize elements
     * with the desired load factor.
     */
    @VisibleForTesting static int chooseTableSize(int setSize) {
        if (setSize == 1) {
            return 2;
        }
        // Correct the size for open addressing to match desired load factor.
        // Round up to the next highest power of 2.
        int tableSize = Integer.highestOneBit(setSize - 1) << 1;
        while (tableSize * DESIRED_LOAD_FACTOR < setSize) {
            tableSize <<= 1;
        }
        return tableSize;
    }
    
    @GwtIncompatible("java.util.BitSet")
    static CharMatcher from(BitSet chars, String description) {
        // Compute the filter.
        long filter = 0;
        int size = chars.cardinality();
        boolean containsZero = chars.get(0);
        // Compute the hash table.
        char[] table = new char[chooseTableSize(size)];
        int mask = table.length - 1;
        for (int c = chars.nextSetBit(0); c != -1; c = chars.nextSetBit(c + 1)) {
            // Compute the filter at the same time.
            filter |= 1L << c;
            int index = smear(c) & mask;
            while (true) {
                // Check for empty.
                if (table[index] == 0) {
                    table[index] = (char) c;
                    break;
                }
                // Linear probing.
                index = (index + 1) & mask;
            }
        }
        return new SmallCharMatcher(table, filter, containsZero, description);
    }
    
    @Override
    public boolean matches(char c) {
        if (c == 0) {
            return containsZero;
        }
        if (!checkFilter(c)) {
            return false;
        }
        int mask = table.length - 1;
        int startingIndex = smear(c) & mask;
        int index = startingIndex;
        do {
            // Check for empty.
            if (table[index] == 0) {
                return false;
                // Check for match.
            } else if (table[index] == c) {
                return true;
            } else {
                // Linear probing.
                index = (index + 1) & mask;
            }
            // Check to see if we wrapped around the whole table.
        } while (index != startingIndex);
        return false;
    }
    
    @GwtIncompatible("java.util.BitSet")
    @Override
    void setBits(BitSet table) {
        if (containsZero) {
            table.set(0);
        }
        for (char c : this.table) {
            if (c != 0) {
                table.set(c);
            }
        }
    }
}
